The earliest time at which serum antibody peaks following administration of influenza virus vaccine in elderly persons is not clearly defined. We compared the time intervals of 2 and 4 weeks after vaccination. A commercial trivalent vaccine containing the hemagglutinins of influenza viruses A/Texas/36/91(H1N1), A/Shangdong/9/93(H3N2), and B/Panama/45/90 was used. The hemagglutination inhibition antibody titers at 2 weeks after vaccination were identical to the hemagglutination inhibition antibody titers at 4 weeks for all three vaccine components.